In general, light olefins are produced by a contact cracking process in which naphtha, a gasoline component obtained from distillation of petroleum at normal pressure, is thermally cracked at 750-850° C. to produce a basic source of petrochemicals, including ethylene, propylene, benzene, and the like.
Especially, ethylene and propylene are used in various fields. Specifically, ethylene is used to produce various useful materials including vinyl chloride, polyethylene, acetic acid, synthetic paints, etc., due to its high reactivity. Propylene is obtained from a liquefied petroleum gas and is used as a raw material for the production of polymer gasoline and other petrochemicals, including isopropyl alcohol, acetone, propylene oxide, propylene glycol, allyl alcohol, glycerol, acrylonitrile, phenol acetone, dodecylbenzene, and the like. Besides, it is used to produce polypropylene through addition polymerization reaction, which is used to prepare a synthetic fiber.
Therefore, development of a process for the production of light olefins with superior yield and selectivity is highly required.
A commonly used technique to improve the selectivity for light olefins is to increase the thermal cracking reactions at higher reaction temperatures. However, at elevated reaction temperatures, the catalysts exhibit inferior long-term stability and feed conversions. Moreover, the high temperature operation can cause an accelerated coke generation on the catalyst that eventually leads to a lower catalytic activity and lower feeding rate of raw material. These factors can limit the production of ethylene and propylene at high yields. Another method to improve selectivity for light olefins is to add a chemical component to modify the activity of the catalyst. One such example is the modification of cracking activity of the ZSM-catalyst by adding phosphorus so as to convert the primary cracking product into C2-C4 light olefins.
At present, many research groups are actively working for the production of light olefins through cracking reactions.
Korean Patent Publication No. 2002-7006467 discloses a highly efficient catalyst composite for the production of light olefins through a conventional fluid catalytic cracking (FCC) process. The catalyst composition selective for olefins is prepared by treating a zeolite catalyst with a compound containing 10 wt % P2O5.
Korean Patent Publication No. 2000-7012182 discloses a cracking process for selectively producing C2-C4 light olefins. First, the gas oil and residual oil feeds are converted in a zeolite based FCC process to obtain low boiling point products that include a hydrocarbon stream that boils in naphtha range. Then, the products are fractionated based on boiling points and only the naphtha boiling range stream is transferred to the second step. A product resultant from the second step reaction is stripped to obtain C2-C4 light olefins with high selectivity. The principle involved in the currently employed contact cracking process is to provide a fluid contact between the thermally pre-treated feed and a heated catalyst for the cracking reaction (fluid contact). The fluid contact technique requires a stripping process for removing coke and other hydrocarbon materials because they may adhere to the catalyst particles during cracking and result in reduced catalytic activity and selectivity.
U.S. Unexamined Patent Application No. 2006/0116544 A1 discloses a catalyst for producing olefins with high selectivity and yield, where a catalyst comprising pentasil type zeolite containing rare earth element is used to improve light olefin selectivity in catalytic cracking.
U.S. Pat. No. 5,026,936 discloses a method for improving selectivity for propylene using a two-step reactor. In the first step, propylene, ethylene and butane are produced in high yield from a hydrocarbon feed through catalytic cracking. After separating the propylene from the product stream, ethylene and butene are further treated in the second step for the production of propylene through olefin metathesis reaction.
U.S. Unexamined Patent Application No. 2007/0083071 discloses a zeolite based process that operates in a fixed or fluidized-bed reactor for the production of light olefins. While supplying a catalyst periodically or continuously into a catalytic cracking reactor, a hydrocarbon feed and steam are fed into the reactor to carry out cracking. The product is separated based on the boiling point, and C2-C3 paraffinic hydrocarbons are recovered for undergoing thermal cracking to produce C2-C3 olefins. C4 or higher hydrocarbons are recycled to the catalytic cracking step along with a feedstock.
U.S. Pat. Nos. 6,106,697 and 6,258,257 disclose a two-step process for improving yield of light olefins. In the first step, gas oil or residual oil (resid) is reacted in a FCC unit containing a large pore zeolite catalyst to produce a mixture of hydrocarbons. In the second step, the hydrocarbons are passed through a reaction zone, a stripping zone and a catalyst regeneration zone to improve selectivity for C2-C4 olefins.
In the aforesaid techniques, FCC process is used for the production of light olefins and the selectivity to light olefins is improved by recycling the undesired product or treating the product stream after the separation of light olefins. However, the cracking reaction is sensitive to various catalysts and process parameters such as reaction temperature, activity of the catalyst and contact time, etc., that requires a precise control to optimize such parameters for selective production of ethylene and propylene.